Bolted Connection Tests of Thin G550 and G300 Sheet Steels (No. R749)

Abstract

Cold formed structural members are fabricated from sheet steels which must meet the material requirements prescribed in applicable national design standards. The Australian / New Zealand Standard for cold-formed steel structures (AS/NZS 4600 (SA/SNZ, 1996)) allows for the use of thin (t < 0.9mm), high strength (fy = 550MPa) sheet steels in all structural sections. However, due to the low ductility exhibited by sheet steels which are cold reduced to thickness the engineer must use a yield stress and ultimate strength reduced to 75% of the minimum specified values. The American Iron and Steel Institute (AISI) Design Specification further limits the use of thin, high strength steels to roofing, siding and floor decking panels. Sheet steels are required to have a minimum elongation capability to ensure that members and connections can undergo small displacements without a loss in structural performance, and to reduce the harmful effects of stress concentrations. The ductility criterion specified in the Australian / New Zealand and North American Design Standards is based on an investigation of sheet steels by Dhalla and Winter, which did not include the thin high strength G550 steels available today. A previous research report entitled Ductility of G550 Sheet Steels in Tension – Elongation Measurements and Perforated Tests (No. R735) detailed the basic material behaviour of G550 sheet steels. It was concluded that the ability of G550 sheet steels to undergo deformation is dependent on the direction of load within the material, where transverse specimens exhibit the least amount of overall, local and uniform elongation. Furthermore, the G550 sheet steels tested for this project do not meet the Dhalla and Winter material requirements regardless of direction, except for the uniform elongation of longitudinal coupon specimens. This report details the findings of bolted connection tests using G550 and G300 sheet steels which range in base metal thickness from 0.42 to 0.60mm. Test specimens were milled from the longitudinal, transverse and diagonal directions of the sheet to determine the degree of anisotropy and its effect on connection capacity and failure type. All specimens failed in one of three distinct modes; end pull-out, bearing or net section fracture. The results of tests completed for this report indicate that the current connection provisions set out in the AS/NZS 4600, AISI and Eurocode 3 Design Standards cannot be used to accurately predict the failure mode or resistance of bolted connections fabricated from thin G550 and G300 sheet steels. It is necessary to incorporate a variable bearing resistance equation which is dependent on the thickness of the connected material, similar to that found in the Canadian CSA-S136 Design Standard. Calculation of the ultimate tensile strength of a bolted connection using the net cross-sectional area and the ultimate material strength, without a stress reduction factor, is accurate and reliable. Bolted connections composed of G550 sheet steels were able to displace to at least 90% of the distance measured for the nominally identical G300 test specimens, which indicates that adequate ductility exists in the tested G550 sheet steels.